This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing an epoxy hydroxy polyether resin extended with a primary amine that forms a high quality, stable electrocoating composition.
The coating of electrically conductive substrates by an electrodeposition process also called an electrocoating process is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an auto body or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired thickness of coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Film forming resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
Cathodic electrocoating compositions, resin compositions, coating baths and cathodic electrodeposition processes are disclosed in Jarabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
The resins used in cathodic electrocoating compositions typically are based on epoxy resins reacted with a dihydric phenol such as bis phenol A which is then chain extended with a polyol and then capped with an amine and reacted with an acid to form a water dispersible or soluble cathodic polymer. With this procedure there is a limit on polymer molecular weight. If the molecular weight is too high, there is an insufficient amount of amine that can be incorporated into the polymer to make it water dispersible. The latter problem can be resolved by chain extending with a diamine. However, gelation problems can occur if the ratio of amine to epoxy groups is not maintained with in an optimum range.
Gelation problems do not occur with the present invention in which the epoxy resin reacted with dihydric phenol is extended with a primary amine and then capped with an amine. The resulting resin or polymer has a sufficient number of amine groups to form a water dispersible polymer even at high molecular weights and allows for the preparation of polymers with a broad range of molecular weights. Electrocoating compositions of these polymers have a wide range of application voltages, improved throw power and wedge performance in conventional cathodic electrocoating baths. If an alkanol amine is used, the resin will have additional hydroxyl groups along the polymer chain which will enhance the crosslinked structure of a resulting coating of the composition.
Throw power of an electrocoating composition is the degree to which an electrodeposited film penetrates and coats the surfaces of recessed interior areas of an automobile or truck body. Electrodeposition of coatings follows the force lines of the electrical field that exists in an electrocoating bath between the cathode and the anode. These force lines diminish as they penetrate into the recessed areas of the auto or truck body and cease to exist when the recessed area is too deep and a coating will not be deposited into such an area.
As automobile and truck body designs change, there is an increased need for electrocoating composition that have increased throw power and that will penetrate and coat recessed areas and there is a continued need for electrocoating compositions that provide improved corrosion resistance. The improved electrocoating composition of this invention has increased throwing power and provides a film having improved corrosion along with other desirable characteristics such as solvent, and chip resistance.